1. Field of the Invention
The present invention relates to a light source driving circuit and module thereof, and more particularly, to a light source driving circuit and module thereof providing a fixed current with reduced temperature dependence.
2. Description of the Prior Art
Applications of light sources utilizing Light Emitting Diodes (LEDs) are more and more common. For example, the back light module of the conventional Liquid Crystal Display (LCD) utilizes a Cold Cathode Fluorescent Lamp (CCFL) as the light source. Nowadays, as efficiency of the LED increases and cost of manufacture decreases, the LED is gradually replacing the CCFL as the light source of the back light module of the LCD.
In a conventional application, a plurality of the LEDs are electrically connected in series for reducing the amount of driving circuits needed and the amount of total driving current. However, due to differences between different manufactures, it is not easy to ensure that parameters of the LEDs (e.g. forward voltage) from different manufactures will be the same. Besides, environmental factors, such as temperature, may affect parameters of the LEDs. For example, as the temperature rises, the forward voltage of the LED falls, so that the size of the current flowing through the LED changes. Since the brightness of the LED is proportional to the current flowing through the LED, when the size of the current flowing through the LED changes, the brightness of the LED changes as well. Therefore, when the temperature rises, the brightness of a plurality of LEDs electrically connected in series also changes. Additionally, a total forward voltage error of a structure of a plurality of LEDs electrically connected in series is equivalent to the sum of the forward voltage error of each LED of the plurality of the LEDs electrically connected in series, and the total forward voltage error differs over different series of LEDs.
Therefore, even if two different series of LEDs are driven with one same voltage, the sizes of the currents respectively flowing through each series of LEDs are different, since there is difference between the total forward voltage errors of the two series of LEDs. The difference between the total forward voltage errors of the two series of LEDs is further increased when the difference between the temperatures in the two series of LEDs increases. In this way, difference between the brightness's of the two series of LEDs is generated. Thus, brightness of an LCD back light module utilizing series of LEDs is easily unbalanced, resulting in Mura on the LCD.